It is known in the art to project images from several projectors onto a curved screen with a high-gain surface to construct an all-surrounding life-like image around a central area. Such a curved, high-gain screen is used to achieve higher luminance of the projected scene when viewed from the central area than is achievable using a plurality of flat screens as the projection surface. Typically, these curved screens have a surface of a spherical or toroidal shape and a surface gain greater than unity so that more light is reflected back to the observer than would otherwise be reflected if the screen had a gain of unity or less. By unity gain, it is meant a surface with characteristics similar to, for example, magnesium oxide or flat, very-white paint, that is, a surface that reflects an incident light ray equally in all directions, and the intensity of a reflected ray is equal irrespective of the angle of observation. By high-gain, it is meant a surface which reflects a light ray preferentially at an angle of reflection equal and opposite the angle of incidence. In measurements typical of the art, the intensity of a reflected ray measured on the specular axis (that is, measured at an angle of reflection equal and opposite the angle of incidence), compared to a similar measurement made on a unity gain screen, is referred to as the screen gain. (Actually, for purposes of defining the bulk surface characteristic called gain, the incident ray is directed normal to the surface; relative gain measured at other angles of incidence varies either slightly or substantially, depending on the type of screen material. However, the foregoing definition is sufficiently accurate for the present discussion). By use of such a high-gain screen, it is known in the art to achieve higher image luminance in the viewing area at the expense of lower luminance outside that area, since screen brightness decreases substantially as the angle of measurement departs from the specular axis. The use of multiple projectors, along with multiple sources of video imagery, serves to increase the sharpness of the image thus created, since each video image, of limited resolution, is then spread over a smaller area. Curved screens and a plurality of projectors are currently used extensively to create large life-like images for various types of simulators.
A problem exists with these curved projection screens in that when the observer is displaced from the central viewing point, there occurs abrupt changes of brightness at the boundaries between the screen segments. This is a common occurrence when multiple observers must view the same image, as for example in an air traffic control tower simulator, where multiple controllers view and interact with the scene of the surrounding airfield and its associated air and ground vehicles. The abrupt light change is due to the fact that a ray of light from one projector on one side of the boundary is reflected closer to the observer than a ray of light just on the other side of the boundary from a different projector. While the human eye is very tolerant of even large changes in luminance that occur gradually across a scene (as is commonly the case with light emanating from any set of projection optics), abrupt discontinuities of even small amounts are immediately and annoyingly apparent. The present invention overcomes this problem by providing screen segments having a surface composed of a multiplicity of ellipsoidal shapes, juxtaposed in a particular fashion.
An ellipse, by definition, has two focal points. An ellipsoidal surface is created by the rotation of an ellipse about the axis containing the two focal points. By the mathematical properties of an ellipsoid, light emanating from one focal point of an ellipsoid onto a specular (mirrored) internal surface is reflected entirely back to the other focal point. Similarly, light projected onto a partially-specular (high-gain) ellipsoidal screen from one focal point is preferentially reflected to the other focal point. By, for instance, arranging the ellipsoidal screens such that their first focal points coincide and projecting the images from the screen's second focal point, respectively, a ray of light on one side of the boundary will reflect through the common first focal point and a ray of light on the other side of the boundary will also reflect through the common first focal point. Accordingly, it is impossible to move farther away from a ray of light reflected from one side of the boundary than from the other. Thus, there will be no abrupt change in brightness across the boundary from any viewpoint.